Aluminum alloys for applications such as wheels and methods of manufacture

ABSTRACT

Aluminum alloys described herein include silicon, iron, copper, manganese, magnesium, and chromium. In various implementations, the aluminum alloys also include one or more of zinc and titanium. Typically, a total amount of iron and manganese in the aluminum alloys is no less than 0.28% by weight and no greater than 0.45% by weight, and the grains in the aluminum alloys have an average grain length of no greater than 6 mm. Aluminum alloy billets can be forged for wheel production at selected temperatures.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a division of U.S. patent application Ser.No. 16/219,536, filed Dec. 13, 2018, which claims priority to U.S.provisional patent application No. 62/617,018, filed Jan. 12, 2018, thedisclosure of both of which are hereby incorporated by reference intheir entirety.

FIELD OF DISCLOSURE

The present disclosure relates to materials, methods, and techniques ofmanufacture for aluminum alloys. Example applications relate to thepreparation and manufacture of aluminum wheels.

INTRODUCTION

Aluminum wheels can experience fatigue and develop cracks in one or morelocations. FIG. 1A is a partial side, cross-sectional view of examplewheel 10. FIG. 1B is a partial front plan view of example wheel 10.Wheel 10 includes rim 12 and disc 16, connected near transition radius20. Rim 12 includes rim drop center 14 and closed side flange 18. Disc16 defines a plurality of hand holes 22. At a mounting portion, disc 16defines concave disc face 24 and convex disc face 26. Typically, wheelcracks from fatigue can occur in a rim drop center, near the closed sideflange, near the transition radius, at the concave disc face, at theconvex disc face, and adjacent to a hand hole.

SUMMARY

Vehicle wheels are made of various materials, such as aluminum alloysand steel. Safety and performance considerations for vehicle wheelsinclude a given wheel's ability to resist fatigue for extended periodsof time. It is particularly challenging to achieve those goals withaluminum wheels, which are designed to be lighter than steel wheels.

Materials, methods and techniques disclosed and contemplated hereinrelate to aluminum alloys. Aluminum wheels manufactured with aluminumalloys disclosed herein, and in accordance with methods and techniquesdisclosed here, exhibit improved performance compared to existingaluminum wheels.

In one aspect, an aluminum alloy is disclosed. The aluminum alloyincludes, by weight: 0.80% to 1.20% silicon; 0.08% to 0.37% iron; 0.35%to 0.55% copper; 0.07% to 0.37% manganese; 0.70% to 1.20% magnesium;0.05% to 0.11% chromium; no more than 0.20% zinc; and no more than 0.05%titanium, and the balance of weight percent comprising aluminum andincidental elements and impurities.

In another aspect, an aluminum wheel having a rim and a disc isdisclosed. The aluminum wheel is formed of an aluminum alloy comprising,by weight: 0.80% to 1.20% silicon; 0.08% to 0.37% iron; 0.35% to 0.55%copper; 0.07% to 0.37% manganese; 0.70% to 1.20% magnesium; 0.05% to0.11% chromium; no more than 0.20% zinc; and no more than 0.05%titanium, and the balance of weight percent comprising aluminum andincidental elements and impurities.

In another aspect, a method for making an aluminum alloy is disclosed.The method includes receiving an aluminum alloy billet and forging thealuminum alloy billet at a temperature no less than 275° C. and nogreater than 460° C. The aluminum alloy billet includes, by weight:0.80% to 1.20% silicon; 0.08% to 0.37% iron; 0.35% to 0.55% copper;0.07% to 0.37% manganese; 0.70% to 1.20% magnesium; 0.05% to 0.11%chromium; no more than 0.20% zinc; and no more than 0.05% titanium, andthe balance of weight percent comprising aluminum and incidentalelements and impurities.

Other aspects of the disclosure will become apparent by consideration ofthe detailed description and accompanying drawings. There is no specificrequirement that a material, technique or method include all of thedetails characterized herein, in order to obtain some benefit accordingto the present disclosure. Thus, the specific examples characterized aremeant to be exemplary applications of the techniques described, andalternatives are possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a partial side, cross-sectional view of a prior art aluminumwheel. FIG. 1B is a partial front plan view of the wheel shown in FIG.1A.

FIG. 2 is an example determination of grain width and grain length in amicrograph.

FIG. 3 shows photographs of experimental results from etching testalloys.

FIG. 4 shows grain structures for various test wheels after etching.

FIG. 5 shows fatigue performance of each of the test wheels shown inFIG. 4.

FIG. 6 shows fatigue performance for test wheel E shown in FIG. 4 fortwo different alloys.

FIG. 7 shows fatigue performance for two test wheels shown in FIG. 4when subjected to ASTM B368 followed by a radial wheel fatigue test.

FIG. 8 shows a tonnage-stroke plot for two forgings comprised ofdifferent alloys.

FIG. 9 shows a flow stress curve plot for the alloys in FIG. 8.

FIGS. 10A, 10B, and 10C show scanning electron micrographs of surfacesof three different billets.

FIGS. 11A, 11B, and 11C show optical micrographs of etched surfaces ofthe billets shown in FIGS. 10A-10C.

FIG. 12 shows grain length for the three alloys in FIGS. 10A-10C whenforged at different temperatures.

FIG. 13 shows a fraction of fine grains in the slope of wheels madeusing the three alloys in FIGS. 10A-10C forged at differenttemperatures.

FIG. 14 shows a fraction of fine grains in the mounting area of wheelsmade using the three alloys in FIGS. 10A-10C forged at differenttemperatures.

FIG. 15 shows a fraction of fine grains in the disc area of wheels madeusing the three alloys in FIGS. 10A-10C forged at differenttemperatures.

FIG. 16 is a micrograph of one of the alloys shown in FIGS. 10A-10C.

FIG. 17 shows the number cycles to failure for wheels made using thethree alloys in FIGS. 10A-10C forged at different temperatures.

DETAILED DESCRIPTION

Aluminum alloys described and contemplated herein are particularly wellsuited for use in aluminum wheel manufacture. For instance, aluminumwheels manufactured using the materials, techniques and methodsdescribed and contemplated herein have improved fatigue performance.Broadly characterized, advantageous properties in aluminum wheelsdisclosed herein can be attributed to grain structures resulting fromselection and performance of components and techniques disclosed herein.

In the following sections, example aluminum alloys are described,including various characteristics of the example aluminum alloys.Example methods of making aluminum wheels are also described. Last, adescription is provided of experimental test results relating to examplealuminum alloys and wheels manufactured with the example aluminumalloys.

I. Example Aluminum Alloys

Aluminum alloys described and contemplated herein can be characterized,for instance, by components, grain structure, or dispersoids. In someembodiments, a total amount of iron and manganese in the aluminum alloysis no less than 0.28% by weight and no greater than 0.45% by weight. Insome embodiments, grains in the aluminum alloys have an average grainlength of no greater than 6 mm.

A. Example Components and Amounts

Aluminum alloy compositions disclosed and contemplated herein includevarious components at various weight percentages, in addition toaluminum. Example components capable of inclusion in aluminum alloysdisclosed and contemplated herein include: silicon (Si), iron (Fe),copper (Cu), manganese (Mn), magnesium (Mg), chromium (Cr), zinc (Zn),and/or titanium (Ti). Without being bound by a particularly theory, itis believed that both Si and Cu increase strength and fatigue resistanceof the aluminum alloy.

In various implementations, aluminum alloys disclosed and contemplatedherein include, by weight, 0.80% to 1.20% silicon; 0.08% to 0.37% iron;0.35% to 0.55% copper; 0.07% to 0.37% manganese; 0.70% to 1.20%magnesium; 0.05% to 0.11% chromium; and the balance of weight percentcomprising aluminum and, in certain instances, incidental elements andimpurities. In various embodiments, aluminum alloys can further compriseno more than 0.20% zinc or no more than 0.05% titanium.

In other implementations, aluminum alloys disclosed and contemplatedherein include, by weight, 0.90% to 1.00% silicon; 0.08% to 0.37% iron;0.40% to 0.50% copper; 0.07% to 0.37% manganese; 1.00% to 1.10%magnesium; 0.05% to 0.11% chromium; and the balance of weight percentcomprising aluminum and, in certain instances, incidental elements andimpurities. In various embodiments, aluminum alloys can further compriseno more than 0.20% zinc or no more than 0.05% titanium.

Aluminum alloys disclosed and contemplated herein can also becharacterized by the total amount of iron and manganese (i.e., the sumof the weight percent of iron and the weight percent of manganese). Invarious embodiments, the total amount of iron and manganese is no lessthan 0.28% by weight. In other embodiments, the total amount of iron andmanganese is no greater than 0.45% by weight. In yet other embodiments,the total amount of iron and manganese is no less than 0.28% by weightand no greater than 0.45% by weight. In still other embodiments, thetotal amount of iron and manganese is no less than 0.30% by weight andno greater than 0.37% by weight.

Incidental elements and impurities in the disclosed alloys may include,but are not limited to, nickel, vanadium, zirconium, or mixturesthereof, and may be present in the alloys disclosed herein in amountstotaling no more than 1%, no more than 0.9%, no more than 0.8%, no morethan 0.7%, no more than 0.6%, no more than 0.5%, no more than 0.4%, nomore than 0.3%, no more than 0.2%, no more than 0.1%, no more than0.05%, no more than 0.01%, or no more than 0.001%.

The alloys described herein may consist only of the above-mentionedconstituents, may consist essentially of such constituents, or, in otherembodiments, may include additional constituents.

B. Grain Structure

Aluminum alloy compositions disclosed and contemplated herein can alsobe characterized by grain structure. As used herein, a “grain” is apancake-shaped distinct crystal in the aluminum alloy, usually having anaspect ratio of length to width of from 5 to 25. Grain size analysis canbe discussed in terms of grain length and grain width, where both grainlength and grain width measurements are average values of the grains. Anexample identification of grain width 202 and grain length 204 is shownin FIG. 2. For this disclosure, grain size is typically measured on thescale of millimeters.

Generally speaking, during forming and heat treating processes, grainsize changes. Grain size changes relate to the types of formingprocesses used and the quantity of dispersoids. Dispersoids are part ofthe chemical composition and are formed from those certain elements(e.g., Fe, Mn, Cr) in the alloys. Dispersoid density can control theresulting grain structure and grain size.

Grain size can be determined using the following method. First, a sampleis metallographically polished with final colloidal silica (0.04 μm)polish. In some instances, the sample is obtained from a disc slopeportion of an aluminum wheel. Then a swab etch with Keller's reagent (2ml HF, 3 ml HCL, 5 ml HNO₃, 190 ml H₂O) is performed for approximately 1minute. Then optical microscopy and grain size measurements can beperformed by ASTM E112 line method in the horizontal (length) andvertical (width) directions.

In implementations where aluminum alloys disclosed and contemplatedherein are used in the manufacture of wheels, grain size impacts wheelfatigue performance. As shown below in experimental examples, wheelfatigue performance improves with alloys having average grain lengths nogreater than 6.0 mm.

Finer grains are typically better for resisting the formation of fatiguecracks. Fine grains can be defined as unrecrystallized grains formedduring casting, unrecrystallized grains with subgrains formed during hotforging, and/or dynamically recrystallized grains formed during hotforging.

Aluminum wheels formed from aluminum alloys disclosed and contemplatedherein have less than 50% by area fraction of fine grains in the discportion of the wheel. In various embodiments, aluminum wheels have lessthan 45%; less than 40%; less than 30%; less than 25%; or less than 20%by area fraction of fine grains in the disc portion of the wheel.

The instant disclosure shows that fretting-fatigue followed by fatiguecrack growth is a common failure mode on the disc faces of wheelsbecause of the vehicle mounting configuration. Medium recrystallizedgrains can demonstrably enhance the resistance of the wheel tofretting-fatigue and fatigue crack growth in these parts of the wheel.Furthermore, medium recrystallized grain structures provide goodresistance to both fatigue crack initiation and fatigue crack growth inother parts of the wheel.

Aluminum alloys forged according to techniques and methods disclosed andcontemplated herein have an average grain length no greater than 6.0 mm.In various embodiments, aluminum alloys disclosed and contemplatedherein have an average grain length no greater than 4 mm. In otherembodiments, aluminum alloys disclosed and contemplated herein have anaverage grain length of no less than 0.4 mm and no greater than 6 mm. Asparticular examples, aluminum alloys disclosed and contemplated hereinhave an average grain length no greater than 5.50 mm; no greater than5.00 mm; no greater than 4.75 mm; no greater than 4.50 mm; no greaterthan 4.25 mm; no greater than 4.00 mm; no greater than 3.75 mm; nogreater than 3.50 mm; no greater than 3.25 mm; no greater than 3.00 mm;no greater than 2.75 mm; no greater than 2.50 mm; no greater than 2.25mm; no greater than 2.00 mm; no greater than 1.75 mm; no greater than1.50 mm; no greater than 1.25 mm; no greater than 1.00 mm; no greaterthan 0.75 mm; no greater than 0.50 mm; or no greater than 0.40 mm.

Grain size can also be determined in terms of average grain width.Aluminum alloys disclosed and contemplated herein typically have a grainwidth no greater than 0.40 mm, even when an average grain length isgreater than 4 mm and less than 6 mm. In some instances, grain width isno greater than 0.30 mm. In other instances, grain width is no less than0.25 mm.

Grain size determination typically includes determining an average grainlength and width of grains within one or more samples. The samples aretwo dimensional sections of a wheel. Grain size can be determined ineach sample and, where multiple samples are used in the determination,the grain sizes may be averaged. As an example, multiple samples alongthe wheel profile are obtained and each sample may have a length of 25mm and a width equal to the wheel thickness.

C. Dispersoids

Aluminum alloy compositions disclosed and contemplated herein can alsobe characterized by dispersoids. The term “dispersoids” is known in theart and, generally, refers to pieces of various alloy components. Forinstance, dispersoids can be iron, manganese, chromium, titanium, and/orsilicon rich intermetallic compounds with various stoichiometry's(Al—Fe—Si, Al—Mn, Al—Cr, Al—Fe(Mn,Cr)—Si, Al—V, Al—Zr, Al—Ti).

Generally speaking, desired grain structures (e.g., grain size anddistribution) are impacted by the number, size and distribution ofdispersoids in the aluminum alloy. For common commercially availablealuminum alloys, the number of dispersoids in the aluminum alloy can berelated to an amount of iron, manganese, or chromium, as well as thehomogenization temperature of the cast billet.

Homogenization methods can be adapted to attain one or more desiredproperties as disclosed and contemplated herein. As an example,homogenization can include slowly heating a billet to a temperaturebetween 550° C. and 575° C. for between 2 hours and 8 hours, followed byfan cooling in air. In some instances, homogenization occurs at atemperature between 550° C. and 560° C. In some instances,homogenization heating occurs for about 4 hours.

In various implementations, dispersoids in the aluminum alloys disclosedand contemplated herein have a distribution of no greater than 0.20 perμm². In other implementations, dispersoids in the aluminum alloysdisclosed and contemplated herein have a distribution of no greater than0.10 per μm². In yet other implementations, dispersoids in the aluminumalloys disclosed and contemplated herein have a distribution of from0.06 per μm² to 0.10 per μm². Distributions below 0.03 per μm² mayresult in excessive coarsening of the grain structure.

In various implementations, dispersoids have an average size of from 230nm to 260 nm. In other implementations, dispersoids have an average sizeof from 230 nm to 250 nm. In yet other implementations, dispersoids havean average size of from 228 nm to 248 nm.

D. Forging

Billets of aluminum alloy compositions disclosed and contemplated hereincan be forged on conventional closed die forging presses to producevarious products. For example, aluminum alloy billets can be forged intoa pre-machined disc portion of a wheel using a closed die forging press.

Aluminum alloy billet forging can occur at various temperatures,assuming atmospheric pressure conditions. Atmospheric pressureconditions mean a pressure of the external environment at the locationat which the process of the present disclosure is operated. As examples,aluminum alloy billets can be forged at a temperature no less than 275°C. and no greater than 460° C. In other implementations, aluminum alloybillets are forged at a temperature no less than 350° C. and no greaterthan 400° C. In yet other implementations, aluminum alloy billets areforged at a temperature no less than 370° C. and no greater than 427° C.or 450° C. In yet other implementations, aluminum alloy billets areforged at a temperature of no less than 275° C. and no greater than 427°C. As specific examples, the cast aluminum alloy billet is forged at atemperature of 260° C., 270° C., 280° C., 290° C., 300° C., 310° C.,320° C., 330° C., 340° C., 350° C., 360° C., 370° C., 380° C., 390° C.,400° C., 410° C., 420° C., 427° C., 430° C., 440° C., 450° C., or 460°C.

Generally, higher forging temperatures provide better die lifelongevity. Alloys and wheels disclosed and contemplated herein can havegrain structures disclosed above at higher forging temperatures.Accordingly, a related benefit of alloys and wheels disclosed herein maybe improved die life longevity.

II. Example Methods of Making Aluminum Wheels

An example method for making an aluminum wheel includes first receivinga cast and homogenized aluminum alloy billet, where the aluminum alloybillet includes one or more properties and components described above.Then the cast aluminum alloy billet is heated to a temperature of from260° C. to 460° C.

After heating the billet, a wheel profile is forged using a conventionalclosed die forging press with one continuous hit to the billet at astrain rate of from 0.0254 to 2.54 cm/cm/sec. During this operation,material within the forging press undergoes dynamic recrystallization.

Following forging of the wheel profile, a rim contour is flow formed ata temperature of from 21° C. to 316° C. Next, solution heat treatment isperformed at a temperature of from 510° C. to 566° C. During solutionheat treatment, material in the disc undergoes static recrystallization,grain growth, or both, to obtain desired grain sizing. In someinstances, material in the disc is approximately 538° C. during the heattreatment.

Other forging processes are possible, such as multistep forgingprocesses. For instance, the example method can include forging apancake, then the bowl shape, and then forging the rim. As anotherexample, the example method can include forging the pancake, then thebowl, and then spin forming the rim.

Then the forge is quenched. After quenching, the forging is aged at atemperature of from 148° C. to 233° C. Last, the wheel is machined fromthe central part of the forging.

III. Experimental Examples

Experimental examples of aluminum alloys disclosed above were made andtested. In some instances, the experimental examples were formed intowheels and compared with existing commercial wheels. In particular,experiments on the alloys and wheels included performance testing suchas fatigue performance determinations, experimental manufacturingmethods, and grain and dispersoid analysis.

A. Experimental Grain Size Development

Experiments were performed with example alloys to evaluate grain sizefor various aluminum wheels. In particular, one objective of theexperiments was to optimize grain size for improved wheel fatigueperformance while using forging temperatures that preferably were nogreater than 427° C. Two different alloys were tested and each alloy wasforged at temperatures of 371° C., 399° C., 427° C. and 454° C. The twoalloys are listed below in Table 1.

TABLE 1 Example alloys tested on closed die traditional forgingoperation. Test Alloy Si Fe Cu Mn Mg Cr Zn Ti 1 1.09 0.21 0.45 0.21 0.950.18 0.01 0.017 2 0.92 0.28 0.46 0.17 1.06 0.19 0.02 0.025

In the experiments, grain size analysis was performed on the disc slopeonly. The grain size is the average length of a grain. The experimentsdid not include collecting data on a fraction of recrystallized andunrecrystallized grains in the disc. Fatigue tests were also performedon each alloy, where fatigue life is an estimate based on Accuride TestStandard CE-006 (which follows SAE J267). The grain size analysis datafor each test alloy is shown in Table 2, below.

TABLE 2 Experimental results for the test alloys in Table 1. Failure wasnoted in Table 2 by the locations where they occurred, namely: CC =concave; HH = hand hole; TR = disc to rim transition under the beadseat. Estimated Forging Grain Fatigue Test Temperature size Life FailureAlloy (° C.) (mm) (cycles) Mode 1 454 25.4 516,667 CC-HH-TR 427 5.33683,333 CC-HH-TR 371 2.03 933,333 CC-HH-TR 2 454 19.3 333,333 CC-HH-TR427 4.83 633,333 CC-HH-TR 399 3.81 966,667 CC-HH-TR 371 1.27 1,100,000CC-HH-TR

As is shown in Table 2 above, generally, as forging temperatureincreases, so too does grain size. Additionally, as grain sizedecreases, estimated fatigue life improves. Without being bound by aparticular theory, it is believed that the transition in performance forboth test alloys at higher temperatures is associated with a combinationof grain coarsening and the stabilization of fine grains.

FIG. 3 shows photographs of experimental results from etching the twotest alloys 1 and 2 at 371° C., 399° C., 427° C., and 454° C. Etchingwas performed by (1) uniform grinding with 120 grit SiC and (2) 5minutes in solution of 300 mL H₂O, 75 g FeCl₃, 450 mL HNO₃, and 150 mLHCl at room temperature. As shown in FIG. 3's photographs of Alloy 1 andAlloy 2 at 454° C., there is grain coarsening and stabilization of finegrains. For alloys forged at temperatures less than 400° C., grains inthe disc portion were less than 4.0 mm long and displayed mediumrecrystallized grain structure. Also, for Test Alloy 2, a desired grainstructure of no greater than 4.0 mm grain length can be achieved whenforging at a temperature of no greater than 399° C.

B. Experimental Performance Compared to Available Wheels

Experiments were performed on wheels with example alloys to comparealloys disclosed and contemplated herein with commercially availablewheels. Specifically, five wheels were analyzed for grain structure andfatigue performance. Each wheel was an industry standard 22.5 inch×8.25inch (57.15 cm×20.955 cm) hub piloted wheel. Compositions and wheelweights for each of the five wheels are provided in Table 3, below.

TABLE 3 Compositions and wheel weights for tested wheels. Test WeightComposition (wt. %) Wheel Standard (kg) Si Fe Cu Mn Mg Cr Zn Ti A AA636118.1  0.6-0.9 0.40 0.20-0.50 0.10-0.20 1.0-1.4 0.10-0.30 0.25 0.15 BAA6061 26.3 0.40-0.8 0.7 0.15-0.40 0.15 0.8-1.2 0.04-0.35 0.25 0.15 CAA6061 21.3 0.40-0.8 0.7 0.15-0.40 0.15 0.8-1.2 0.04-0.35 0.25 0.15 DAA6061 20.4 0.40-0.8 0.7 0.15-0.40 0.15 0.8-1.2 0.04-0.35 0.25 0.15 EAA6099 18.1  0.8-1.2 0.7 0.10-0.7  0.10-0.40 0.7-1.2 0.04-0.35 0.25 0.10

FIG. 4 shows grain structures for each test wheel after etching, whereF=fine grains (those less than 0.4 mm), M=medium grains (those between0.4 mm and 4 mm), and C=coarse grains (those greater than 4 mm). Etchingwas performed by (1) uniform grinding with 120 grit SiC and (2) 5minutes in solution of 300 mL H₂O, 75 g FeCl₃, 450 mL HNO₃, and 150 mLHCl at room temperature. FIG. 4 shows that Test Wheel E is the only TestWheel having medium grain size structure. In addition, Test Wheel Eshows medium grain size structure throughout the disc portion of thewheel.

FIG. 5 shows fatigue performance of each of the test wheels listed inTable 3, where the fatigue performance is an average of at least twowheels. Wheel weights (in kg) are listed above each bar graph. Fatigueperformance was obtained using Accuride CE-006 (which follows SAE J267).Test wheel E has the best fatigue performance of the five test wheels,and significantly better performance than test wheel A that is the samewheel weight.

FIG. 6 shows fatigue performance for test wheel E but manufactured withtwo different alloys (AA6061 and AA6099). These wheels were forged atsimilar temperatures and both exhibited medium recrystallized grainsizes of less than 4 mm. Fatigue performance was obtained using AccurideCE-006 (which follows SAE J267). The values shown in FIG. 6 are averagesfor 12 or more wheels. Improved performance of wheel E with AA6099 overwheel E with AA6061 can be attributed to, without being bound by anyparticular theory, high strength from Cu, Si, and Mg; better grainstructure control with higher Mn; and/or improved resistance to fatiguecrack propagation with higher Mg.

FIG. 7 shows fatigue performance, as it specifically relates tocorrosion behavior, for test wheel E and test wheel A when subjected toASTM B368 (a 12 hr copper acetic acid salt spray (CASS) test) followedby a radial wheel fatigue test per Accuride CE-006. Comparing theresults of test wheel A in FIG. 5 with the results shown in FIG. 7,without being bound by any particular theory, it appears that thefailure mode of test wheel A was intensified when the wheel was exposedto CASS-fatigue. Again without being bound by any particular theory, itappears that poor grain structure control influenced wheel fatigueperformance for both corroded and non-corroded wheels.

C. Experimental Example of a Closed Die Forging Process for AA6061 andAA6099

An experimental example evaluated required forces needed to form a wheelprofile during the forging operation. In particular, a wheel profile wasformed using an alloy known as AA6061 and an alloy known as AA6099. Thealloy AA6061 included 0.4%-0.8% silicon, 0.15%-0.4% copper, 0.15%manganese, 0.8%-1.2% magnesium, 0.04%-0.35% chromium, 0.7% iron, 0.25%zinc, 0.15% titanium, and the balance aluminum, by weight percent. Thealloy AA6099 included 0.8%-1.2% silicon, 0.1%-0.7% copper, 0.1%-0.4%manganese, 0.7%-1.2% magnesium, 0.04%-0.35% chromium, 0.7% iron, 0.25%zinc, 0.1% titanium, and the balance aluminum, by weight percent.

FIG. 8 shows a tonnage—stroke plot depicting the required forces neededto form the wheel part during this step in the operation. These datawere obtained from the forging press during forming of the part, andshow that the forces required to form the forged part are notsignificantly different when comparing AA6099 and AA6061, therebyshowing that the formability is similar for the two alloys. This isfurther supported by the flow stress curves as shown in FIG. 9, whichcan be used to characterize the formability of a specific material.Whereas the tonnage curve includes other variables such as dietemperature, forging lubricant and die geometry, flow stress datatypically do not include these variables and the flow stress data wereobtained by taking small compression samples from cast billet stock,heating them to the forming temperature and testing them on a laboratoryscale press. These data further support the conclusion that theformability and flow stress for alloys AA6099 and AA6061 are comparableunder similar hot deformation parameters.

D. Experimental Grain Structure Analysis for Different Levels of Cr andMn

Three different alloy compositions were tested with varying levels of Crand Mn. It will be appreciated that other components' weight percentagesare slightly different because of normal production variance. The threecompositions, having low Cr and Mn, medium Cr and Mn, and high Cr andMn, relatively, are provided in Table 4 below. The Medium alloy has thecomposition of AA6099.

TABLE 4 Compositions of “low Cr and Mn”, “medium Cr and Mn”, and “highCr and Mn” alloys tested. Alloy Si Fe Cu Mn Mg Cr Zn Ti Low 0.9  0.260.43 0.07 0.98 0.05 0.01 0.05 Medium 0.95 0.26 0.45 0.17 1.02 0.18 0.020.03 High 0.87 0.25 0.43 0.32 0.98 0.20 0.01 0.03

Each of the low, medium, and high alloys were forged at five differentforging temperatures between 371° C. and 482° C. on conventional closeddie forging presses. Specifically, the alloys were forged at 371° C.,399° C., 427° C., 454° C., and at 482° C. Trial data suggests that flowstress and press tonnage increases as Cr and Mn concentrations increase.Spin, heat treat, and machining processes were the same for each of thelow, medium, and high alloys. The following analyses were performed:three radial fatigue tests on wheels for each alloy/temperaturecombination; macrostructure; tensile properties; and microstructure atdisc mounting face, disc slope and rim drop center.

Table 5 below provides various characteristic data of the billets foreach alloy.

TABLE 5 Billet characteristic data for the alloys in Table 4. Al-Fe-SiMg₂Si Surface Grain Size Vol. Size Vol. ISZ Shell size (μm) Fract. (μm)Fract. depth Zone Alloy (μm) Porosity Inclusions L W (%) L W (%) (μm)(μm) Low 52 <1% 100 mm 3.0 1.7 1.5 3.1 1.9 0.3 122 381 Al-Fe-Si Med 54<1% None 3.2 1.7 1.4 4.0 2.3 0.7 208 353 High 54 <1% (1) 75 × 3.4 1.92.2 2.3 1.3 0.4 108 321 25 μm

TABLE 6 Chemical composition of the elements comprising the Al-Fe-Si2^(nd) phase constituent particles from Table 5. at. % Al Fe Cu Si Cr MnFe + Cr + Mn Low 71.4 13.8 0.9 12.6 0.5 0.7 15.1 Medium 79.4  9.4 1.2 6.2 1.2 2.6 13.3 High 76.9  4.9 0.5  9.6 2.5 5.6 13.0

As the manganese and chrome increase in the alloy, the fraction of theseelements increases inhomogenously in the Al—Fe—Si, 2^(nd) phaseconstituent particles, and more of these particles form (higher volumefraction in Table 5). It is theorized that this causes a reduction inavailable Si for the formation of the strengthening phases that arecomprised of Mg—Si and Al—Mg—Si—Cu particles. Further, it is theorizedthat when the amounts of manganese and chromium in the alloy are low,the volume fraction of Al—Fe—Si is lower, but the concentration of Si inthe phase is higher. It is theorized that this will limit the freesilicon needed to form the strengthening precipitates.

Dispersoids of each billet were also measured. The dispersoids weremeasured in the center of grains. Table 7 shows the experimentalresults. Spacing is calculated as 1/distribution. The “high” billetdispersoid analyzed was disc-shaped, with a length of 289 nm and a widthof 138 nm.

TABLE 7 Dispersoids data for the billets in Table 5. DispersoidDistribution Spacing Billet size (nm) (#/μm²) (μm) Low 248 0.06 16.7Medium 226 0.29 3.4 High 214 0.50 2.0

FIGS. 10A, 10B, and 10C show micrographs of surfaces of the low, medium,and high billets. FIGS. 10A, 10B, and 10C were obtained using a scanningelectron microscope (SEM) set at: extra high tension (EHT) voltage of20.0 kV, working distance (WD) of 5.0 mm, magnification 20,000×.Dispersoids 402, 404, and 406 are labeled in FIGS. 10A, 10B, and 10C.Not all dispersoids are labeled. It can be seen from FIGS. 10A, 10B, and10C that as the Cr and Mn increases, the quantity of dispersoids alsoincreases.

FIGS. 11A, 11B, and 11C show optical micrographs of etched surfaces ofthe low, medium, and high billets. FIGS. 11A, 11B, and 11C were obtainedusing the following method. First, a sample was metallographicallypolished with final colloidal silica (0.04 μm) polish. Then a swab etchwith Keller's reagent (2 ml HF, 3 ml HCL, 5 ml HNO₃, 190 ml H₂O) wasperformed for approximately 1 minute. Then optical microscopy and grainsize measurements were performed by ASTM E112 line method in thehorizontal (length) and vertical (width) directions.

During etching, the dispersoids disappear and create pitting. It can beseen from FIGS. 11A, 11B, and 11C that the amount of pitting increasesas the Cr and Mn increases.

Grain size was determined for each of the three alloys at the fivedifferent forging temperatures, shown in the graph in FIG. 12.Generally, grain length increased with higher preheat temperatures foreach of the three alloys. Acceptable radial fatigue at the hand hold andthe edge of nut (EON) positions was observed for alloys having averagegrain lengths greater than 0 mm and no greater than 4 mm.

A percent of fine grains for each of the three alloys was alsodetermined at three wheel locations. FIG. 13 shows a fraction of finegrains in the slope of the wheel. FIG. 14 shows a fraction of finegrains in the mounting area of the wheel. Without being bound by aparticular theory, it appears that having a high percentage of finegrains in the mounting area resulted in worse fatigue performance. Forinstance, acceptable radial fatigue in the mounting area was observedwhen the fraction of fine grains in the mounting area was no greaterthan 50%. FIG. 15 shows a fraction of fine grains in the disc portion ofthe wheel. FIG. 16 is a micrograph of one of the alloys, where grain 902is an example of medium grain and grain 904 is an example of fine grain.

Radial fatigue cycles-to-failure experiments were also performed onwheels for the three alloys forged at five different temperatures. FIG.17 is a logarithmic plot showing the number of cycles-to-failure forwheels made with the three alloy compositions forged at five differenttemperatures.

Area 1002 shows forging temperatures and composition ranges where closedside flange cracks were observed, possibly because of coarse grains.Area 1004 shows forging temperatures and composition ranges wheremounting face cracks were observed, possibly because the grains were toofine. Area 1006 shows forging temperatures and composition ranges wherehand hole cracks were observed, possibly because the grains were toocoarse.

As an example, closed side flange cracks were observed in the “medium”alloy wheel forged at 371° C. In the “low” alloy wheel forged at 371°C., medium recrystallized grains were observed and there were no closedside flange cracks after the same number of cycles that caused the“medium” alloy at 371° C. to crack. Edge of nut cracks were observed inthe “high” alloy forged at 427° C. Hand hole cracks were observed inboth the “low” alloy forged at 454° C. and the “medium” alloy forged at371° C. Based on these experimental data, without being bound by aparticular theory, it appears that forging the “low” alloy attemperatures of at least 371° C. but less than 427° C. provides the bestfatigue performance.

E. Experimental Grain Structure Analysis for Different Levels of Fe andMn

Experimental example composition alloys were manufactured and tested toanalyze different levels of Fe and Mn and to evaluate resulting grainstructures. More specifically, five different test compositions weremanufactured and forged at a temperature of 427° C. Grain size analysiswas performed on each test composition, which included determining anaverage grain length and an average grain width, both in millimeters(mm). The five test compositions and the determined average grain sizesare provided in Table 8, below.

TABLE 8 Grain size analysis for different amounts of Fe + Mn.Composition (wt. %) Grain size Fe + (mm) Test Si Fe Cu Mn Mg Cr Zn Ti MnLength Width 1 0.90 0.26 0.43 0.07 0.98 0.05 0.01 0.05 0.33 3.31 0.31 20.89 0.19 0.43 0.06 1.02 0.07 0.02 0.02 0.25 4.24 0.40 3 0.88 0.18 0.410.06 1.02 0.06 0.02 0.02 0.24 4.64 0.42 4 0.88 0.16 0.36 0.06 0.96 0.070.01 0.03 0.22 5.10 0.42 5 0.86 0.16 0.36 0.06 0.96 0.07 0.01 0.03 0.224.71 0.39

FIG. 18 is a plot showing grain length (mm) as a function of Fe+Mncontent in weight percentage (wt. %). Without being bound by aparticular theory, it appears that in order to obtain alloys havingaverage grain lengths no greater than 4.0 mm, Fe+Mn wt % should be noless than 0.28 wt %. Experimental results also indicate that when Fe+Mncontent is more than 0.45 wt %, undesirable fine grains are stabilized.

F. Experimental Grain Structure Analysis for Higher Temperatures

Experimental example wheels were manufactured and tested for grain sizeand fatigue performance. Wheels having the composition in Table 9 wereforged at 399° C., 427° C., and 454° C. A total amount of Fe and Mn inthe alloy was 0.28 wt. %.

TABLE 9 Example alloy tested at various forging temperatures. Si Fe CuMn Mg Cr Zn Ti Component 0.91 0.18 0.39 0.10 1.02 0.06 0.01 0.02 (wt. %)

In the experiments, grain size analysis was performed on the disc slopeonly. The grain size is the average grain length and average grainwidth. Fatigue tests were also performed on each alloy, where fatiguelife is an estimate based on Accuride Test Standard CE-006 (whichfollows SAE J267). The grain size and fatigue life data for each wheelalloy is shown in Table 10, below.

TABLE 10 Experimental radial fatigue life results for wheelsmanufactured with the test alloy in Table 9. Disc slope Radial Forginggrain size (mm) Fatigue Temperature Aspect Life (° C.) Length Widthratio (cycles) 454 5.97 0.38 16 1,283,333 427 5.46 0.30 18 1,483,333 3994.45 0.29 15 1,766,666

As shown in Table 10, wheels having average grain lengths in the discslope greater than 4.00 mm but less than 6.00 mm display improved radialfatigue life. Thinner grains appear to provide good fatigue performancefor wheels forged at temperatures at or above 427° C.

For the recitation of numeric ranges herein, each intervening numbertherebetween with the same degree of precision is contemplated. Forexample, for the range of 6-9, the numbers 7 and 8 are contemplated inaddition to 6 and 9, and for the range 6.0-7.0, the numbers 6.0, 6.1,6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, and 7.0 are contemplated. Foranother example, when a pressure range is described as being betweenambient pressure and another pressure, a pressure that is ambientpressure is expressly contemplated.

It is understood that the foregoing detailed description andaccompanying examples are merely illustrative and are not to be taken aslimitations upon the scope of the disclosure. Various changes andmodifications to the disclosed embodiments will be apparent to thoseskilled in the art. Such changes and modifications, including withoutlimitation those relating to the chemical structures, substituents,derivatives, intermediates, syntheses, compositions, formulations, ormethods of use, may be made without departing from the spirit and scopeof the disclosure.

We claim:
 1. A method for making an aluminum alloy, the methodcomprising: receiving an aluminum alloy billet, comprising, by weight:0.80% to 1.20% silicon; 0.08% to 0.37% iron; 0.35% to 0.55% copper;0.08% to 0.37% manganese; 0.70% to 1.20% magnesium; 0.05% to 0.11%chromium; no more than 0.20% zinc; and no more than 0.05% titanium, andthe balance of weight percent comprising aluminum and incidentalelements and impurities; and forging the aluminum alloy billet at atemperature no less than 275° C. and no greater than 460° C.
 2. Themethod according to claim 1, wherein forging the aluminum alloy billetincludes using a closed die forging press to form a pre-machined discportion of the wheel.
 3. The method according to claim 1, thetemperature being no less than 370° C.
 4. The method according to claim3, the temperature being no greater than 425° C.
 5. The method accordingto claim 2, wherein a total amount of iron and manganese is no less than0.28% by weight and no greater than 0.45% by weight.